Trapezoid may be the finest folk ensemble in America today, but they still find time every year to play at nursing homes in Virginia and West Virginia. It was at a nursing home in the band's home base of Elkins, W. Va., that Lorraine Duisit got the inspiration for a song.

"Near the end of our shows," she relates, "Ralph, our bassist, always does a clogging demonstration. Well, after this one show, an old woman in a wheelchair came up to him and said, 'I'll bet you thought you were dancing alone up there. But you had a dancing partner, because I was dancing with you in my heart.'

"That struck me so deeply," she continues. "It reminded me of this title I had thought of for a song: 'Wild Bird in a Purple Plum.' The vitality in that woman's heart reminded me of the beauty of the song from a bird hidden in a tree."

When Trapezoid plays that song in concert, group founder Paul Reisler knocks out the tricky rhythm on a dumbek, a ceramic Saharan drum, and Ralph Gordon repeats his clogging demonstration.

This ability to combine poetic imagery, an African drum and Appalachian folk dancing all into one number is typical of Trapezoid's considerable reach.

"People are often amazed at the different things we can pull in and still make work," Reisler notes. "They don't seem to have any trouble recognizing our sound, though. We don't have the problem that some eclectic musicians have.

"Some groups will play a swing tune and then do an old-timey tune, and it will sound like two different bands. That's because they take each song directly from the source.

"When we learn a song we like, we sit around and play it a lot. We change things quite a bit -- even melodies and harmonies. As a result, all these songs have a particular sound that's ours."

When they performed Eric Schoenberg's "Devrah's Delight" at the Wolf Trap Barns last November, the sustained notes of Freyda Epstein's fiddle, Gordon's cello and Duisit's bowed psaltery created an eerie reverie that evoked the deep woods without sounding like any piece of traditional music. When they did Barbara Keith's "A Bramble and a Rose," Reisler's hammered dulcimer rang out over the thick vocal harmony between Epstein's earthy alto and Duisit's more ethereal soprano.

The four Trapezoid albums contain material as diverse as Louis Jordan's jump blues "Ain't Nobody Here but Us Chickens," the African tribal chant "Kora," the traditional bluegrass tune "God Bless That Moonshiner," Jon Hendricks' jazz ballad "Malice Towards None," and Si Kahn's contemporary folk song "Sailing Alone."

"Recently we were at a very traditional festival in Kentucky," Duisit recounts, "where we had a lengthy introduction that explained we weren't a normal traditional folk band. Yet the crowd had no problem hearing the tradition in our music. It seems truer to my roots to take a song like 'Wagoner's Lad' and take liberties with it than to make it sound as close to the original as possible.

"My parents are French, and I come from a European classical background, so learning to play frailing banjo doesn't have much to do with my roots. There's a strength in classical music that very much has to do with its arranged sound. Yet I was drawn as a youngster to the folk music of Joan Baez and Joni Mitchell, because it had a freedom that let one's personality come through. I'm seeking a synthesis of the strength of classical arrangements and the personableness of folk music."

Reisler was a folk guitarist from Alexandria when he met Sam Rizzetta at the Philadelphia Folk Festival 17 years ago. Rizzetta was demonstrating the then obscure hammered dulcimer, a trapezoid-shaped sounding board under strings struck by two wood mallets. Reisler was so entranced with the overlapping sounds of the resonant strings that he eventually moved to Elkins and began building the instruments with Rizzetta. To illustrate what the instrument could do, Reisler, Rizzetta and two apprentices formed a hammered dulcimer quartet that took its name from the instrument's shape. As the band became more and more a full-time venture, the part-timers dropped out. Gordon, who grew up in New Jersey, joined in 1977 from a West Virginia old-time band. Duisit, a Charlottesville folk singer, joined in 1978. Epstein, a Boston folkie, completed the current quartet in 1980.

Trapezoid has just finished recording its fifth album, "Cool of the Day," which Sugar Hill Records will release in October. It will contain Jean Ritchie's title tune, several Duisit originals, "A Bramble and a Rose" and Ferron's "I Never Was to Africa," the latter arranged by Duisit's new husband, Tom Espinola, with discreet tastes of synthesizer and percussion.

"We do a medley on the album of my song 'Rainfall' and a Civil War song called 'The Rose Is Gone,' " Duisit says, "where we use harmonic singing, a method that the monks once used, where the harmony notes seem to float above the song. Tom's arrangement of the Ferron song resembles the trance music of Steve Reich or Philip Glass. These are just things we wanted to try; we have to keep stretching in new directions."

Trapezoid will preview selections from the album at the Birchmere tonight. The quartet then returns to its home base of Elkins for the annual Augusta Heritage Folk Festival at Davis & Elkins College.

The four members will lead a workshop there on "The Art of Performance/The Business of Music" Aug. 4-9, while their friends Sam Rizzetta, Howard Levy and Tom Espinola will lead workshops on the hammered dulcimer, harmonica and mandolin, respectively. They will all perform together the evening of Aug. 6.